The invention relates to a method of manufacturing glass bodies, in which method a thixotropic suspension, the starting material for the glass body, is used to form a porous green body which is subsequently subjected to a purification step in a heated gaseous phase and then sintered to form a glass body.
The invention further relates to arrangements for carrying out such a method as well as to the use of the glass bodies manufactured by the method in accordance with the invention.
The method mentioned in the opening paragraph is particularly suitable for the manufacture of preforms for optical waveguides.
In order to homogenize very viscous moulding materials which can be formed without previous dehydration prior to the moulding process, use is made in the art of ceramics, dependent upon the viscosity, of roller mills (.eta..apprxeq.10.sup.3 Pa.multidot.s), kneading machines (.eta..apprxeq.10.sup.4 Pa.multidot.s) or extruders (.eta..apprxeq.10.sup.5 . . . 10.sup.6 Pa.multidot.s).
Using mixers which are known in the art to mix and homogenize the aqueous, very viscous moulding materials containing quartz-glass particles is disadvantageous because the rollers, kneading blades, or barrel extruders are mostly made of steel alloys. The hardness of the quartz-glass powder (Mohs' scale of hardness: quartz.apprxeq.7, steel.apprxeq.5) inevitably leads to abrasion, and as the kneading times increase to a considerable amount of impurities in the moulding material caused by the presence of iron, chromium, nickel etc. which are difficult to remove or at least substantially increase the cost of purifying the green bodies formed from the moulding material. These impurities are particularly disadvantageous in the case of sintering of green bodies to form quartz-glass tubes or quartz-glass rods as cladding material or core material for optical waveguides, because small concentations in the ppb-range already adversely affect the transmission properties of optical waveguides.
An additional disadvantage of the use of kneading devices and extruders is that each time only a small part of the moulding material to be processed is sheared. As highly concentrated aqueous substances of microdispersed quartz-glass particles exhibit an extremely high degree of thixotropy in comparison with other known ceramic substances, the viscosity of the substance is lower only in the sheared areas which, consequently, are the only locations where the substance is thoroughly mixed. In order to obtain homogeneous substances, the substance to be processed must be removed from the dead angles and pressed into the shearing zone, which operation entails additional costs.
In kneading using a three-roll mill, each volume element is sheared equally (if it is at all drawn in by the rolls), yet, the substances which are milled to form a thin film quickly lose humidity and, thus, change their composition and viscosity if no additional action is undertaken to counteract these unwanted effects.
Consequently, the known methods of processing very viscous ceramic substances have the disadvantage of being very time-consuming, and besides they necessitate lengthy purification processes when very pure quartz-glass bodies, such as for the manufacture of optical waveguides, are to be formed.